Learning Kotlin - An Android Developers Journey

This presentation was presented at the Kotlin Johannesburg meetup.

Learning a new programming language is a daunting thought. It is not an overnight accomplishment, where the next day you are suddenly well versed in the new language. Rebecca is in the process of learning Kotlin. She has often felt like the biggest imposter trying to learn this new language that is growing in popularity. She has had thoughts like: “But there are so many experts out there, I’ll never know as much as them.”, “How can I start writing Kotlin when I don’t know all the features there are in the language?”. The biggest lesson she has learned about learning Kotlin is to just write it. In this talk, she will cover some of the nifty features she has explored in Kotlin, as well as the many different ways she has gone about trying to learn this new language, slowly overcoming her imposter syndrome whilst writing Kotlin code.

Find Rebecca Franks on Twitter: https://twitter.com/riggaroo

Transcript

1. Learning Kotlin An Android Developer’s Journey Rebecca Franks @riggaroo
2. What is Kotlin?
3. What is Kotlin?
4. Why Learn Kotlin?
5. My Journey
6. I saw general hype about Kotlin...
7. I watched a video...
10. I wrote an article...
13. Then I forgot about it..
14. Google I/O happened
15. Google I/O 2017 - Kotlin Support Announcement
16. In my mind
17. I tried Kotlin Koans...
18. try.kotlinlang.org
19. I tried an Online Course
20. Udemy
21. I bought a Kotlin book
23. I bought another Kotlin Book…
25. I decided to CMD +ALT+ SHIFT + K a few files
27. Eventually I decided to just try write Kotlin code
30. You don’t need to know all the features to write Kotlin code. Learn as you go.
31. Kotlin Things
32. Semicolons - there are none *well unless you want them
33. var / val
34. var = variable value
35. val = constant value
36. var temperature : Int = 1 // has type Int val numberMembers = 5 //automatically inferred type Int val percentage = 80 / 100 percentage = 30 //does not compile var percentage = 80 / 100 percentage = 30 //compiles
37. var temperature : Int = 1 // has type Int val numberMembers = 5 //automatically inferred type Int val percentage = 80 / 100 percentage = 30 //does not compile var percentage = 80 / 100 percentage = 30 //compiles
38. var temperature : Int = 1 // has type Int val numberMembers = 5 //automatically inferred type Int val percentage = 80 / 100 percentage = 30 //does not compile var percentage = 80 / 100 percentage = 30 //compiles
39. var temperature : Int = 1 // has type Int val numberMembers = 5 //automatically inferred type Int val percentage = 80 / 100 percentage = 30 //does not compile var percentage = 80 / 100 percentage = 30 //compiles
40. Nullability
41. The Kotlin Type system distinguishes between references that can hold null and those that can not
42. Avoid NullPointerException *well unless you want them
43. //No Nulls Allowed var average : Double = 1.0 average = 2.0 average = null
44. //No Nulls Allowed var average : Double = 1.0 average = 2.0 average = null //Allows nulls var average : Double? = 1.0 average = 2.0 average = null
45. // Double !! -> NullPointerException val l = name!!.length // Safe Call ?. with chains val name = bob?.department?.head?.name // Elvis Operator val length = b?.length ?: -1
46. // Double Bang !! -> NullPointerException val l = b!!.length // Safe Call ?. with chains val name = bob?.department?.head?.name // Elvis Operator val length = b?.length ?: -1
47. // Double Bang !! -> NullPointerException val l = b!!.length // Safe Call ?. with chains val name = bob?.department?.head?.name // Elvis Operator ?: val length = b?.length ?: -1
48. val name : String? = "hello" name?.let { //Now name is not null at this point textView.text = name } // There are whole bunch more - .with{} .run{} - with slight differences // Can also just do a normal null check!
49. lateinit
50. class EventsActivity : AppCompatActivity() { private lateinit var recyclerViewAdapter: SimpleAdapter override fun onCreate(savedInstanceState: Bundle?) { // Other initialization stuff recyclerViewAdapter = SimpleAdapter(/*..*/) } }
51. lazy
52. val lazyValue: String by lazy { println("computed!") "Hello" } fun main(args: Array<String>) { println(lazyValue) println(lazyValue) } computed! Hello Hello
53. when
54. when (s) { 1 -> print("x == 1") 2 -> print("x == 2") else -> { // Note the block print("x is neither 1 nor 2") } }
55. enum class Currency { GBP, ZAR; } when (currency) { Currency.GBP -> doSomething() Currency.ZAR -> doAnotherThing() }
56. enum class Currency { GBP, USD, ZAR; } when (currency) { Currency.GBP -> doSomething() Currency.ZAR -> doAnotherThing() }
57. enum class Currency { GBP, USD, ZAR; } when (currency) { Currency.GBP -> doSomething() Currency.ZAR -> doAnotherThing() else -> doNothing() }
58. Functions
59. Functions are first-class citizens
60. fun calculateSum(a: Int, b: Int) : Int { return a + b } fun calculateSum(a: Int, b: Int) = a + b
61. fun calculateSum(a: Int, b: Int) : Int { return a + b } fun calculateSum(a: Int, b: Int) = a + b
62. Extension Functions
63. fun String.hello(): String = "Hello " + this // use the new hello() method val hi = "Kotlin !".hello() Hello Kotlin ! fun TextView.getFloatValue() : Float { return 0f } val float = textViewHello.getFloatValue()
64. fun String.hello(): String = "Hello " + this // use the new hello() method val hi = "Kotlin !".hello() Hello Kotlin ! fun TextView.getFloatValue() : Float { return 0f } val num = textViewHello.getFloatValue()
65. Default Values
66. fun calculateSum(a: Int = 2, b: Int) : Int { return a + b } val sum = calculateSum(b = 3) 5 //Can be done on Constructors as well as functions
67. Higher Order Functions & Lambdas
68. val exampleArray = arrayListOf(3,324,5,6,3,2,6,8,1,2,4,3,2) exampleArray.filter { it % 2 == 0 } [324, 6, 2, 6, 8, 2, 4, 2] val numberList = arrayListOf(1,2,3,4,5,6,7,8,9) numberList.map { it * 2 } .filter { it % 3 == 0} [6, 12, 18]
69. val exampleArray = arrayListOf(3,324,5,6,3,2,6,8,1,2,4,3,2) exampleArray.filter { it % 2 == 0 } [324, 6, 2, 6, 8, 2, 4, 2] val numberList = arrayListOf(1,2,3,4,5,6,7,8,9) numberList.map { it * 2 } .filter { it % 3 == 0} [6, 12, 18]
70. fun calcPercentage(value: Int, onComplete: (output : Double) -> Unit) { val calculation = value / 100.0 onComplete(calculation) } //called in onCreate or wherever calculatePercentage(2) { Log.d("MainActivity", "Calculation Complete: $it") } Calculation Complete: 0.02
71. fun calcPercentage(value: Int, onComplete: (input : Double) -> Unit) { val calculation = value / 100.0 onComplete(calculation) } //called in onCreate or wherever calcPercentage(2) { Log.d("MainActivity", "Calculation Complete: $it") } Calculation Complete: 0.02
72. fun <T> executeWork(job: () -> T) : T { Log.d(TAG, "Work started") val result = job() Log.d(TAG, "Work Fin") return result } val number = executeWork { 100 * 5 } Log.d(TAG, "Result: $number") val stringCalc = executeWork { "Rebecca".plus(" Hello") } Log.d(TAG, "String: $stringCalc") Work started Work Fin Result: 500 Work started Work Fin String: Rebecca Hello
73. fun <T> executeWork(job: () -> T) : T { Log.d(TAG, "Work started") val result = job() Log.d(TAG, "Work Fin") return result } val number = executeWork { 100 * 5 } Log.d(TAG, "Result: $number") val stringCalc = executeWork { "Rebecca".plus(" Hello") } Log.d(TAG, "String: $stringCalc") Work started Work Fin Result: 500 Work started Work Fin String: Rebecca Hello
74. fun <T> executeWork(job: () -> T) : T { Log.d(TAG, "Work started") val result = job() Log.d(TAG, "Work Fin") return result } val number = executeWork { 100 * 5 } Log.d(TAG, "Result: $number") val stringCalc = executeWork { "Rebecca".plus(" Hello") } Log.d(TAG, "String: $stringCalc") Work started Work Fin Result: 500 Work started Work Fin String: Rebecca Hello
75. Classes
76. class Admin : User { val name: String val age: Int constructor(name : String, age : Int){ this.name =name this.age = age } fun isAdult(): Boolean { return age >= 18 } }
77. class Admin(val name: String, val age: Int) : User { fun isAdult(): Boolean { return age >= 18 } }
78. class Admin(val name: String, val age: Int) : User { fun isAdult() = age >= 18 }
79. Data Classes
80. // data classes data class Admin(val name: String, val age: Int) // Automatically generates: - toString() - hashCode() / equals() - copy()
81. Coroutines *Experimental
82. Simplify Async Programming Lightweight thread *Spin up 1mil coroutines easily
83. Coroutines are computations that can be suspended without blocking a thread. https://kotlinlang.org/docs/tutorials/coroutines-basic-jvm.html
84. //onCreate job = launch(background) { val result = requestData() launch(UI) { textViewHello.text = result } } //suspend function suspend fun requestData(): String { delay(3000) return "DONE" } //onDestroy job.cancel()
85. //onCreate job = launch(background) { val result = requestData() launch(UI) { textViewHello.text = result } } //suspend function suspend fun requestData(): String { delay(3000) return "DONE" } //onDestroy job.cancel()
86. //onCreate job = launch(background) { val result = requestData() launch(UI) { textViewHello.text = result } } //suspend function suspend fun requestData(): String { delay(3000) return "DONE" } //onDestroy job.cancel()
87. https://kotlinlang.org/docs/tutorials/c oroutines-basic-jvm.html
88. Android KTX
89. github.com/android/android-ktx
90. A set of Kotlin extensions for Android
91. //Before sharedPreferences.edit() .putBoolean("key", value) .apply() //After - with Android KTX sharedPreferences.edit { putBoolean("key", value) }
92. //Before view.viewTreeObserver.addOnPreDrawListener( object : ViewTreeObserver.OnPreDrawListener { override fun onPreDraw(): Boolean { viewTreeObserver.removeOnPreDrawListener(this) actionToBeTriggered() return true } }) //After - with Android KTX view.doOnPreDraw { actionToBeTriggered() }
93. val (a,r,g,b) = Color.MAGENTA val drawable = customDrawable.toBitmap() canvas.withTranslation(x, y) { // block } val (year, month, day) = LocalDate.now()
94. The difficult bits *for me...
95. Mocking - All classes in Kotlin are final unless you tell it otherwise (ie mark it as open ) - Workarounds but seem unnecessary https://antonioleiva.com/mockito-2-kotlin/ http://hadihariri.com/2016/10/04/Mocking-Kotlin-With-Mockito/
96. Tooling IntelliJ is great… but it doesn’t write all the code. Battled with Lambda syntax, it sometimes doesn’t fill in all the blanks nicely Solution - Write it in Java, then copy paste the java into your Kotlin file and it’ll auto convert
97. Nothing, Unit & Any Nothing - never returns - ie infinite loop or exception throwing Unit - default return type for a function Any - Default Super Type (similar to Java’s Object type) http://natpryce.com/articles/000818.html
98. Concerns • Packaging another library in your app (size 9kbs when proguarded - not much) • Another barrier to entry for new developers • Java 8 works with Android.. (not all features ie Stream API only available in 24+) • Certain Java plugins not supported (yet) - Sonarqube https://discuss.kotlinlang.org/t/sonarqube-support/3657
99. Resources
100. Online Courses O’Reilly Introduction to Kotlin YouTube Search “Kotlin on Android” https://www.youtube.com/results?search_query=Kotlin+on+Android
101. Practise Kotlin Koans https://try.kotlinlang.org Kotlin REPL Kotlin Android Codelab
102. try.kotlinlang.org
105. Books • Kotlin in Action - Svetlana Isakova - Dmitry Jemerov • Kotlin for Android Developers - Some common patterns etc
106. Online Community kotlinweekly.net talkingkotlin.com reddit.com/r/kotlin
107. Just start writing Kotlin code
108. Thank you! Questions? Rebecca Franks @riggaroo